This invention relates to television signal synchronizing apparatus and especially to apparatus for synchronizing the television signals from two or more sources of video information; for example, two or more video tape recorders.
In monochrome TV, two separate sources of video information are considered phased together when their respective horizontal and vertical sync pulses are coincident, provided that for vertical sync first and second field sync is also observed. The basic repetition rate is therefore the picture field frequency divided by two, f.sub.V /2. When two signals are so phased, cutting or fading from one source to the other can be carried out without disturbing the synchronization of receivers.
In the case of NTSC, the respective horizontal and vertical sync pulses of two video sources must, of course, be coincident, but, in addition, there is a further requirement in that the two color subcarrier frequencies must be phased together. In practice, this can be done by controlling the local subcarrier frequency with an APC loop which compares the bursts of the two subcarrier signals.
In PAL, the same coincidence of sync pulse and subcarrier phases must be made as in NTSC with the additional requirement that the PAL square wave (f.sub.H /2) of each signal source must also be in phase.
The PAL standards, as defined by the CIRR standards, XII Plenary Assembly 1970, Vol. V, Part 2, Report No. 407-1 specify that the R-Y component of chroma be phase inverted on alternate television lines in synchronism with the PAL square wave. In the case where a PAL video source is not locked to a master PAL square wave as, for example, in a prerecorded video tape, it is necessary to know, on a continuous basis, the chroma designation of each line of the recorded PAL video signal as a positive R-Y or negative R-Y in order to synchronize the recorded PAL video signal chroma component to the master system reference f.sub.H /2.
Known apparatus for detecting the line-by-line color phase requires the detection of a two-line component indicating the R-Y polarity in the error signal from a time base corrector which utilizes a reference subcarrier for comparison purposes. Alternatively, a resonant circuit tuned to the subcarrier and referenced to the leading edge of horizontal sync may be used to make the comparison with the line-by-line burst phase. The present invention eliminates the prior art necessity of a reference subcarrier-related signal for comparison.